Recently, LEDs have come into use as light sources for lighting in lieu of fluorescent lamps. For example, an LED lamp which is shaped like a conventional fluorescent lamp of a straight tube type is disclosed in Japanese Patent Application Publication No. 2009-43447. The LED lamp includes: a light source block having a structure in which a plurality of LEDs are mounted on a mounting substrate of a band plate shape; a glass tube of a straight tube type which stores the light source block therein; pin bases that block up both ends of the glass tube; and terminal pins that project from a side surface of the caps to feed a power to the light source block. Such an LED lamp is freely detachably mounted in a lamp socket provided in a dedicate illumination apparatus and is lit by the power (DC power) being supplied via the lamp socket from an LED lighting device that is mounted on the illumination apparatus.
Moreover, as a conventional LED lighting device, a power source device for lighting is disclosed in Japanese Patent Application Publication No. 2010-287499 (JP2010-287499A). The conventional example includes a power source unit which includes a DC/DC converter of a flyback type, a control circuit (control IC) that controls a switching element of the power source unit, and first and second auxiliary power source circuits that supply a control power to the control circuit.
An input voltage of the power source unit is a ripple voltage which is a full-wave rectified voltage of an AC voltage of a commercial AC power source. The power source unit supplies a direct-current (DC) voltage which is stepped down from the input voltage to an LED. The first auxiliary power source circuit includes a first electrolytic capacitor charged by an input voltage of the power source unit via a current-limiting resistor, and generates a control power with the electric charge charged in the first electrolytic capacitor.
On the other hand, the second auxiliary power source circuit includes a second electrolytic capacitor charged by an induced electromotive force which is induced to an auxiliary coil of a transformer that forms the power source unit and generates a control power with the electric charge charged in the second electrolytic capacitor. In addition, the power source unit further includes a diode of which an anode is connected with a positive electrode of the first electrolytic capacitor and a cathode is connected with a power source terminal of the control circuit and a positive electrode of the second electrolytic capacitor. Moreover, an electrical discharge from the second electrolytic capacitor to the first electrolytic capacitor is prohibited.
In the conventional example disclosed in JP2010-287499A, when the control power is supplied from the first auxiliary power source circuit immediately after supplying a power, and a voltage between both ends of the second electrolytic capacitor exceeds a voltage between both ends of the first electrolytic capacitor owing to the rise in an output voltage of the power source unit, a control power is supplied from the second auxiliary power circuit. In addition, since the operation of the first auxiliary power circuit is stopped while the control power is supplied by the second auxiliary power circuit, the power loss due to the current-limiting resistor can be suppressed.
However, as for light-emitting diode chips used for LED lamps, there are many kinds which differ in forward voltage. Therefore, even if LED lamps output a same optical power, their rated voltages (load voltages) are different when they use different kinds of light-emitting diode chips or include different numbers of light-emitting diode chips. For example, the rated voltage of a 20-type light-emitting diode is specified in a wide range of 22.5 volts to 47.5 volts in JEL801 “L-shape pin base GX16t-5-attached tubular LED lamp system (for general lighting)” which is standardized by Japan Electric Lamp Manufactures Association (JELMA). That is, the output voltage of the LED lighting device that lights such an LED lamp needs to cover the wide range of voltage of from 22.5 volts to 47.5 volts.
Accordingly, when the second auxiliary power circuit supplies the control power of the control circuit, the winding number of an auxiliary coil needs to be increased when the rated voltage Vf of the LED lamp is low (e.g., Vf=22.5 volts) and the control voltage Vcc of the control circuit is high (e.g., Vcc=15 volts). However, when the winding number of the auxiliary coil is increased and, as a result, the ratio of a primary coil and the auxiliary coil approximates 1:1, a problem arises in which the effect of the coupling coefficient of the primary coil and the auxiliary coil remarkably appears and an oscillatory waveform of a frequency higher than an operating frequency is superimposed on the waveform of the electric current that flows to a switching element connected with the primary coil.
On the other hand, when the first auxiliary power supply circuit supplies the control power for the control circuit, especially in a case in which a power factor improving circuit (a step-up copper circuit) is used, a circuitry part which can endure a high voltage of hundreds of volts needs to be used. This leads to an increase in the manufacturing cost. Moreover, there is also another problem in that the power loss increases due to the current-limiting resistor.